Activation of T lymphocytes during an immune response requires physical contact between T cells and antigen-presenting cells (APCs) that results in activation of the antigen-specific T cell receptor (TCR). This cell-cell contact is mediated by the LFA-1 integrin. TCR signaling rapidly increases the functional activity of the LFA-1 integrin, thus enhancing T-APC contacts required for the transmission of the full array of signals required for optimal T cell proliferation and differentiation. Integrin activation is one of the earliest functional responses of a naove T cell to TCR stimulation, and impaired T-APC contact results in inefficient immunity. The adapter protein ADAP is a critical mediator of TCR signaling to integrins. ADAP-/- T cells exhibit impaired conjugate formation with antigen-laden APCs, and also exhibit defective proliferation and cell cycle entry following TCR stimulation. TCR signaling to integrins requires the constitutive association of ADAP with the adapter SKAP55. Regulation of integrin function has been proposed to be the basis for the impaired proliferative response of ADAP-/- T cells. However, we recently identified a novel function for ADAP in regulating NF-kB and JNK activation via TCR-inducible interactions of ADAP with the CARMA1 adapter and the TAK1 kinase that does not involve SKAP55 or the regulation of integrin function. Discrete binding sites on ADAP for SKAP55, CARMA1 and TAK1 have been identified, providing an opportunity to define the contribution of these distinct ADAP-dependent signaling responses to T cell activation and differentiation. In this competing renewal application, we propose to test the hypothesis that ADAP segregates into functionally and biochemically distinct signaling complexes that independently control TCR sensitivity via regulation of integrin-dependent between T cells and APCs, and regulate the activation of CARMA1-dependent signaling pathways. In Aim 1, we will define the unique contribution of ADAP-dependent regulation of integrins and ADAP-dependent regulation of CARMA1-dependent signaling to T cell activation and proliferation. In Aim 2, we will test the function of ADAP in regulating the interaction of T cells with APCs during the initiation of an immune response in vivo, and define the structural elements within ADAP that regulate T cell interaction dynamics, proliferation and differentiation in vivo. To complete these aims, we will utilize ADAP-deficient mice, adenoviral approaches for reconstituting ADAP expression in naive ADAP-/- T cells, adoptive transfer methodologies for visualizing T cell immune responses in vivo, and two-photon microscopy to image T-APC interactions in vivo in real time. This proposal will provide novel insights into the signaling events critical for T cell function, and provide the foundation for enhancing therapeutic modalities targeted at modulating the immune response.